Method of making vertical diode structures

ABSTRACT

A method of making a vertical diode is provided, the vertical dioxide having associated therewith a diode opening extending through an insulation layer and contacting an active region on a silicon wafer. A titanium silicide layer covers the interior surface of the diode opening and contacts the active region. The diode opening is initially filled with an amorphous silicon plug that is doped during deposition and subsequently recrystallized to form large grain polysilicon. The silicon plug has a top portion that is heavily doped with a first type dopant and a bottom portion that is lightly doped with a second type dopant. The top portion is bounded by the bottom portion so as not to contact the titanium silicide layer. For one embodiment of the vertical diode, a programmable resistor contacts the top portion of the silicon plug and a metal line contacts the programmable resistor.

This application is a divisional of U.S. patent application Ser. No.09/505,953, filed on Feb. 16, 2000, which is a divisional of U.S. patentapplication Ser. No. 09/150,317, filed on Sep. 9, 1998 now U.S. Pat. No.6,194,746, which is a divisional of U.S. patent application Ser. No.08/932,791, filed on Sep. 5, 1997, now U.S. patent application No.5,854,102, which is a continuation of U.S. patent application Ser. No.08/609,505, filed on Mar. 1, 1996 now abandoned, all of the foregoingbeing incorporated herein by reference. Two additional applications thatare divisional applications of U.S. patent application Ser. No.09/505,953, filed on Feb. 16, 2000, are filed concurrently with thepresent application.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to vertical diodes and more specificallyto vertical diodes with low series resistance formed on a silicon wafer.

2. The Relevant Technology

One of the common trends in the electronics industry is theminiaturization of electronic devices. This trend is especially true forelectronic devices operated through the use of semiconductor microchips.Microchips are commonly viewed as the brains of most electronic devices.In general, a microchip comprises a small silicon wafer upon which canbe built thousands of microscopic electronic devices that are integrallyconfigured to form electronic circuits. The circuits are interconnectedin a unique way to perform a desired function.

With the desire to decrease the size of electronic devices, it is alsonecessary to decrease the size of the microchip and electronic devicesthereon. This movement has increased the number and complexity ofcircuits on a single microchip.

One common type of electronic device found on a microchip is a diode. Adiode functions as a type of electrical gate or switch. An ideal diodewill allow an electrical current to flow through the diode in onedirection but will not allow an electrical current to flow through thediode in the opposite direction. In conventional diodes, however, asmall amount of current flows in the opposite direction. This isreferred to as current leakage.

Conventional diodes are typically formed from a silicon material that ismodified through a doping process. Doping is a process in which ions areimplanted within the silicon. There are two general types of dopants:P-type dopants and N-type dopants. P-type dopants are materials thatwhen implanted within the silicon produce regions referred to as holes.These holes can freely accept electrons. In contrast, N-type dopants arematerials that when implanted within silicon produce extra electrons.The extra electrons are not tightly bound and thus can easily travelthrough the silicon. In general, a diode is formed when a material dopedwith a P-type dopant is connected to a material doped with an N-typedopant.

Conventional diodes are configured by positioning the two opposing dopedmaterials side by side on a microchip. This side by side positioning,however, uses a relatively large amount of surface space on themicrochip. As a result, larger microchips are required.

Furthermore, for a diode to operate, each side of the diode must have anelectrical connection that either brings electricity to or from thediode. The minimal size of each side of the diode is in part limited inthat each side must be large enough to accommodate an electricalconnection. Since conventional diodes have a side by side configurationwith each side requiring a separate electrical connection, the abilityto miniaturize such diodes is limited. In addition, the requirement ofhaving side by side electrical connections on a single diode increasesthe size and complexity of the microchip.

Attempts have been made to increase the efficiency and current flow ratethrough a diode so as to speed up the microchip. In one attempt toaccomplish this end, one of the sides of the diode is heavily doped andthe other side of the diode is lightly doped. The lightly doped sidelimited the current, and the heavily doped side increased the reversebias leakage. Thus, such a configuration produces minimal gain.

Other attempts have been made to decrease the resistance in the abovediscussed diode by increasing the dopant concentration on the lightlydoped side of the diode. As the dopant concentration is increased,however, current leakage in the diode increases. In turn, the currentleakage decreases the current efficiency and functioning of themicrochip.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide improveddiodes and their method of manufacture.

Another object of the present invention is to provide improved diodesthat use a minimal amount of surface area on a microchip.

Still another object of the present invention is to provide improveddiodes that are easily connected to other electronic devices of anintegrated circuit.

Also another object of the present invention is to provide improveddiodes having improved current flow and efficiency.

It is another object of the present invention to provide improved diodeshaving a heavily doped area and a lightly doped area with minimalresistance and current leakage.

Yet another object of the present invention is to provide improveddiodes that can be selectively sized.

Finally, another object of the present invention is to provide improveddiodes having a minimal cost.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

In order to achieve the above objectives and in accordance with theinvention as claimed and broadly described herein, a vertical diode isprovided on a silicon wafer. The silicon wafer is doped with a firsttype of dopant and has an exposed surface. A vertical diodeincorporating features of the present invention is manufactured byinitially highly doping the exposed surface of the silicon wafer with asecond type of dopant to form an active region.

Next, the active region is covered by a refractory metal silicide layer,preferably titanium silicide. The silicide layer has a relatively lowresistance and, thus, ultimately decreased the resistance through thevertical diode. An insulation layer, such as silicon dioxide, is thenformed over the refractory metal silicide layer. The insulation layer isformed using conventional oxidation deposition processes. A conventionalmasking and etching process is used to etch a diode trench through theinsulation layer so as to expose a portion of the refractory metalsilicide layer. The diode trench is defined by an interior surface whichcontacts the refractory metal silicide layer.

The diode trench is next filled with amorphous silicon which is thenlightly doped with the second type of dopant. The amorphous siliconforms a silicon plug within the diode trench. The silicon plug has abottom portion contacting the refractory metal silicide layer and a topportion separated from the refractory metal silicide layer by the bottomportion.

The amorphous silicon is next heated to recrystallize the amorphoussilicon into large grain polysilicon. The second portion of the siliconplug, now converted into polysilicon, is then heavily doped with thefirst type of dopant. The doping is performed by ion implantationfollowed by a heat treatment, such as RTP, for activation of the dopant.Finally, a metal contact is secured to the top portion of the siliconplug to complete the vertical diode.

Since the diode has a vertical formation, use of the surface area on thesilicon microchip is minimized. Furthermore, as there is only oneconnection point on top of the diode, the diode is easier to connect toother elements and is easier to size.

In one alternative embodiment, a programmable resistor is positionedbetween the metal contact and the top portion of the silicon plug. Theprogrammable resistor comprises chalcogenide material and barriermaterials. One preferred barrier material is titanium nitride. Theprogrammable resistor allows the diode to have memory characteristics.

In yet another alternative embodiment, a second refractory metalsilicide layer is formed on the interior surface of the diode trenchprior to deposition of the amorphous silicon. This second silicidelayer, which is preferably titanium silicide, is used to decrease theresistance through the lightly doped end of the inventive diode.

Formation of the second refractory metal silicide layer is preferablyaccomplished by initially depositing a layer of sacrificial polysiliconon the interior surface of the diode trench. A blanket layer of titaniumor some other refractory metal is then deposited over the polysiliconlayer. Sintering is then used to form the two layers into titaniumsilicide.

The present invention also discloses other embodiments of novel verticaldiodes having low series resistance. For example, in one embodiment thesilicon wafer has an oxide layer with a hole etched therethrough tocommunicate with a silicon substrate. The silicon substrate is dopedwith a P-type dopant. The hole in the oxide layer is filled with apolysilicon plug that is heavily doped with an N-type dopant. Theresulting silicon wafer is heated to a temperature sufficient to cause aportion of the dopants in the polysilicon plug to diffuse into thesilicon substrate. As a result, a diode is formed having a junctionlocated within the silicon substrate. If desired, a programmableresistor and metal contact can then be positioned on top of thepolysilicon plug.

Finally, in yet another alternative embodiment, a vertical diode isformed by initially lightly doping a silicon substrate with a P-typedopant to form an active region. An oxide layer is then deposited overthe silicon substrate. Holes are etched through the oxide layer down tothe active region in the silicon substrate. The entire silicon wafer isthen positioned within a reactor chamber where an epitaxial siliconlayer is grown at the bottom of the holes against the active region.Once the epitaxial silicon layer is grown, the remaining portion of theholes are filled with a polysilicon plug that is heavily doped with anN-type dopant. The silicon wafer is then exposed to an elevatedtemperature that causes a portion of the dopants in the polysilicon plugto diffuse into a top portion of the epitaxial silicon layer. As aresult, a diode is formed wherein the junction is positioned within theepitaxial silicon layer. As before, a programmable resistor and metalcontact can then be positioned on top of the polysilicon plug.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered by referenceto specific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a cross-sectional elevation view of a silicon wafer having anoxide layer covering a portion thereof;

FIG. 2 is a cross-sectional elevation view of the silicon wafer in FIG.1 having an active region;

FIG. 2A is a cross-sectional elevation view of the silicon wafer in FIG.2 having a refractory metal deposited thereon so as to cover the activeregion;

FIG. 2B is a cross-sectional elevation view of the silicon wafer in FIG.2A having the refractory metal partially removed and converted to asilicide layer over the active region;

FIG. 3 is a cross-sectional elevation view of the silicon wafer in FIG.2B having an insulation layer covering the silicide layer;

FIG. 4 is a cross-sectional elevation view of a plurality of diodetrenches extending through the insulation layer of FIG. 3 and to thesilicide layer;

FIG. 5 is a cross-sectional elevation view of the silicon wafer in FIG.4 having amorphous silicon filling the diode trenches;

FIG. 6 is a cross-sectional elevation view of the silicon wafer in FIG.5 having a planarized surface to form silicon plugs filling the diodetrenches;

FIG. 7 is a cross-sectional elevation view of the silicon wafer in FIG.6 wherein each of the silicon plugs comprises a top portion doped with afirst type dopant and a bottom portion doped with a second type dopant;

FIG. 8 is a cross-sectional elevation view of the silicon wafer in FIG.7 having a programmable resistor and a metal contact;

FIG. 8A is an enlarged side view of the programmable resistor in FIG. 8and a diode combination;

FIG. 9 is a cross-sectional elevation view of the silicon wafer in FIG.8A without the programmable resistor material;

FIG. 10 is a cross-sectional elevation view of the silicon wafer shownin FIG. 6 having a polysilicon layer and a refractory metal layer;

FIG. 11 is a cross-sectional elevation view of the silicon wafer in FIG.10 wherein the polysilicon layer and the refractory metal layer areconverted to a single silicide layer;

FIG. 12 is a cross-sectional elevation view of the silicon wafer in FIG.11 having a layer of amorphous silicon;

FIG. 13 is a cross-sectional elevation view of the silicon wafer in FIG.12 after planarization;

FIG. 14 is a cross-sectional elevation view of the silicon wafer in FIG.13 having an oxide layer and a photoresist layer each having a channelpositioned therethrough to each of a plurality of silicon plugs, each ofthe silicon plugs having a top portion and a bottom portion;

FIG. 15 is a cross-sectional elevation view of the silicon wafer in FIG.14 having a programmable resistor and a metal contact;

FIG. 16 is a cross-sectional elevation view of the silicon wafer in FIG.14 having a metal deposited on each of the silicon plugs;

FIG. 17 is a cross-sectional elevation view of the silicon wafer in FIG.16 having a programmable resistor and metal contact and further showinga connection plug for delivering electricity to the inventive diodes;

FIG. 18 is a cross-sectional elevation view of an alternative embodimentof a silicon wafer having an oxide layer and polysilicon layer;

FIG. 19 is a cross-sectional elevation view of the silicon wafer in FIG.18 having an active region formed by dopants diffused from thepolysilicon layer;

FIG. 20 is a cross-sectional elevation view of another alternativeembodiment of a silicon wafer having a pair of active regions separatedby field oxide regions;

FIG. 21 is a top plan view of the silicon wafer shown in FIG. 20;

FIG. 22 is a cross-sectional elevation view of the silicon wafer shownin FIG. 20 having an epitaxial silicon layer and a polysilicon layer;

FIG. 22A is a cross-sectional elevation view of the silicon wafer shownin FIG. 22 wherein the epitaxial silicon layer has been doped bydiffusion from the polysilicon layer;

FIG. 23 is a side cross-sectional elevation view of the silicon wafer inFIG. 22A showing the formation of a pair of adjacent diodes;

FIG. 24 is a top plan view of the silicon wafer in FIG. 23;

FIG. 25 is a cross-sectional elevation view of the silicon wafer shownin FIG. 23 having a programmable resistor and metal contact positionedat the top of each diode;

FIG. 25A is a cross-sectional elevation view of the silicon wafer inFIG. showing a strapping configuration over the diodes;

FIG. 26 is a cross-sectional elevation view of an alternative embodimentof a silicon wafer having an active region;

FIG. 27 is a cross-sectional elevation view of the silicon wafer in FIG.26 having a doped polysilicon layer positioned thereon;

FIG. 28 is a cross-sectional elevation view of the silicon wafer in FIG.27 having a plurality of oppositely doped columns;

FIG. 29 is a cross-sectional elevation view of the silicon wafer in FIG.28 having an oxide layer covering the columns with contacts extendingthrough the oxide layer down to the columns; and

FIG. 30 is a cross-sectional elevation view of the silicon wafer in FIG.29 showing the inventive diodes having a strapping with programmableresistors shown as well.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to improved vertical diodes and methodsfor manufacturing such diodes on a silicon wafer. Depicted in FIG. 1 isa layered wafer 10 used in constructing one embodiment of a verticaldiode incorporating features of the present invention. Layered wafer 10comprises a conventional silicon wafer 12 overlaid by an oxide layer 14.Silicon wafer 12 is doped with a first type dopant. As used in thespecification and appended claims, the terms “first type dopant” and“second type dopant” can each refer either to an N-type dopant or aP-type dopant. However, once a convention is selected for manufacturingof a diode, the convention must be maintained. That is, either all firsttype dopants must be N doped and all second type dopants P doped, or allfirst type dopants must be P doped and all second type dopants N doped.

Oxide layer 14 is shown as having a hole 16 formed therethrough toexpose a contact surface 15 on wafer 12. Hole 16 can be formed using anyconventional masking and etching processes. As shown in FIG. 2, anactive region 18 is formed in wafer 12 by heavily doping wafer 12through contact surface 15 with a second type dopant.

Once active region 18 is obtained, a refractory metal silicide layer 17,seen in FIG. 2B, is formed over active region 18. As depicted in FIG.2A, refractory metal silicide layer 17 is formed by initially depositinga refractory metal layer 19 over layered wafer 10 so as to contact andcover active region 18. Refractory metal layer 19 preferably has athickness ranging from about 500 Angstroms to about 1000 Angstroms.Deposition of refractory metal layer 19 may be accomplished bysputtering, chemical vapor deposition, or most other process by whichsuch metals are deposited. Refractory metal layer 19 is preferablyformed of titanium (Ti), however, other refractory metals such astungsten (W), tantalum (Ta), cobalt (Co), and molybdenum (Mo) can alsobe used.

Next, rapid thermal processing (RTP) is used to sinter refractory metallayer 19. The sintering step is performed in a nitrogen (N₂) richenvironment at a temperature ranging from about 500° C. to about 650° C.For the formation of titanium silicide, the preferred exposure timeranges between about 10 seconds to about 20 seconds.

As a result of the sintering, the top or exposed portion of refractorymetal layer 19 reacts with the surrounding nitrogen to form a nitride,for example, TiN. In contrast, the portion of refractory metal layer 19adjacent to active region 18 reacts with the silicon to form refractorymetal silicide layer 17 seen in FIG. 2B. The composition of refractorymetal silicide layer 17 is dependent on the refractory metal used. WhereTi is used, refractory metal silicide layer 17 is TiSi₂. Other silicidesthat can be formed include, by way of example, WSi₂ TaSi₂, CoSi₂, andMoSi₂.

Next, layered wafer 10 is etched to remove the refractory metal nitridebut leave refractory metal silicide layer 17. The resultingconfiguration, as shown in FIG. 2B, has refractory metal silicide layer17 both contacting and covering active region 18.

Once refractory metal silicide layer 17 is obtained, an insulation layer20 is formed over layered wafer 10 so as to cover refractory metalsilicide layer 17. Insulation layer 20 is preferably silicon dioxide(SiO₂) formed through a deposition oxidation process. Although mostconventional deposition oxidation processes will work, high temperature,thermal oxidation processes are preferably not used. The use of hightemperatures during oxidation can drive the dopant out of active region18. Accordingly, it is preferred that the deposition oxidation processbe performed at a temperature ranging from about 750° C. to about 900°C. Insulation layer 20 is next planarized by either chemical-mechanicalpolishing (CMP) or photoresist etchback, as shown in FIG. 3.

As depicted in FIG. 4, a diode trench 24 is next formed throughinsulation layer 20 using conventional masking and etching processes.Diode trench 24 extends through insulation layer 20 and accessesrefractory metal silicide layer 17 in contact with active region 18.Diode trench 24 is further defined by an interior surface 25 whichcomprises opposing sidewalls 26 formed from insulation layer 20 and afloor 28 formed from a portion of refractory metal suicide layer 17. Asshown in FIG. 4 and each of the other figures, a plurality of diodetrenches 24 and subsequent diode structures can simultaneously be made.Since each of the diode trenches and the diodes formed therein aresubstantially identical, however, reference will only be made to asingle structure.

As shown in FIG. 5, the next manufacturing step entails filling eachdiode trench 24 with amorphous silicon. The filling step is accomplishedby initially depositing an amorphous silicon layer 36 over layer wafer10, thereby simultaneously covering insulation layer 20 and eithersubstantially or completely filling each diode trench 24. Amorphoussilicon layer 36 is preferably deposited using an open or closed tubedeposition process that simultaneously deposits and dopes amorphoussilicon layer 36. Once amorphous silicon layer 36 is deposited, theamorphous silicon is lightly doped with the same dopant (second typedopant) as active region 18.

In a preferred embodiment, chemical mechanical polishing is next used toremove a portion of amorphous silicon layer 36 such that insulationlayer 20 is exposed. As shown in FIG. 6, this step results in layeredwafer 10 having an exposed planarized surface 37. Furthermore, eachdiode trench 24 is left being filled with a silicon plug 38. Siliconplug 38 contacts refractory metal silicide layer 17 at floor 28 and isbounded by insulation layer 20 at side walls 26. Chemical mechanicalpolishing is the preferred method for removing amorphous silicon layer36 since it eliminates the need for masking. Alternatively, photoresistetchback can be used for partial removal of amorphous silicon layer 36.

Amorphous silicon has a higher current leakage than either polysiliconor epitaxial silicon. To minimize leakage, one embodiment of thepreferred invention recrystallizes the amorphous silicon intosubstantially large grain polysilicon after the amorphous silicon isdeposited.

Amorphous silicon recrystallizes into large grains of polysilicon whenit is exposed to elevated temperatures in a range between about 550° C.to about 650° C. over a period of time. In general, the crystal grainsize increases as the exposure time increases at a constant temperature.As the size of the grains increase, the surface area of the grainsdecrease per unit volume. Accordingly, the number of boundary layersbetween the grains also decrease per unit volume. As the grainboundaries decrease, the current leakage decreases. Time and energyrequired for recrystalization, however, increases manufacturing costs.

To optimize the above factors, the amorphous silicon is preferablyheated at a temperature ranging from about 450° C. to about 550° C. withabout 500° C. to about 530° C. being more preferred. The amorphoussilicon is preferably exposed to the above temperatures for a period oftime ranging from about 18 hours to about 48 hours with about 18 hoursto about 30 hours being more preferred. As a result, the amorphoussilicon is converted to a polysilicon preferably having an average grainsize ranging from about 0.3 microns to about 0.8 microns with about 0.4microns to about 0.6 microns being more preferred.

In the preferred embodiment, the amorphous silicon is heated in ahydrogen rich environment. The hydrogen fills the dangling bonds at thegrain boundaries, thereby helping to anneal the grains together. Inturn, annealing of the grains helps to further decrease the currentleakage.

To further optimize the effect of increasing the size of the silicongrains, it is also preferred to minimize the width, designated by theletter “w” in FIG. 5, of diode trench 24. That is, by minimizing thewidth “w” of diode trench 24, the number of grains needed to fill diodetrench 24 is also decreased, thereby decreasing the number of grainboundaries. In part, however, the width “w” of diode trench 24 islimited by the required current needed to pass through the diode forprogramming. As a result, diode trench 24 preferably has a width in arange between about 0.3 microns to about 0.8 microns with about 0.4microns to about 0.6 microns being more preferred.

Formation of the large grain polysilicon is preferably accomplisheddirectly after deposition of amorphous silicon layer 36 but, as in analterative process, can be performed after chemical-mechanical polishingof amorphous silicon layer 36.

Once silicon plug 38 is formed and exposed as discussed above, aphotoresist layer 41 is positioned over planarized surface 37, as shownin FIG. 7. Photoresist layer 41 is patterned to independently exposesilicon plug 38. Ion implantation is then used to heavily dope a topportion 42 of silicon plug 38 with the first type dopant. Photoresistlayer 41 is then removed. As a result of the above step, silicon plug 38comprises top portion 42 which is separated from refractory metalsilicide layer 17 by a bottom portion 44. Bottom portion 44 isidentified as the portion of plug 38 that was not subjected to the ionimplantation of the first type of dopant. As such, bottom portion 44 isstill lightly doped with the second type of dopant.

After the ions from the first type of dopant have been implanted intotop portion 42 of silicon plug 38, the dopant must be activated. In thepreferred embodiment, the dopant is activated using RTP. The RTP cyclepreferably heats top portion 42 to a temperature in a range betweenabout 950° C. to about 1100° C., over a time period between about 5seconds to about 20 seconds. Other conventional annealing processes canalso be used to activate the dopant.

In one embodiment incorporating features of the present invention, theinventive diode can be used as a memory device. In this embodiment, asshown in FIG. 8, a programmable resistor 46 is next positioned over andin contact with top portion 42 of silicon plug 38. As used in thespecification and appended claims, the term “programmable resistor”defines a plurality of alternatively stacked layers of memory material,such as ovonic or chalcogenide, and barrier material, such as titaniumnitride. In the preferred embodiment, there is a layer of chalcogenidematerial surrounded by two to five layers of barrier material.

A metalization step forms a metal contact 48, as shown in FIG. 8, incontact with programmable resistor 46 to form a vertical diode 50. Metalcontact 48 is formed using the same steps as discussed above, namely,deposition, masking, and etching.

FIG. 8A discloses one embodiment of programmable resistor 46 situated ona substrate 12 with a layer of carbon or titanium nitride layer 47superadjacent to substrate 12. Situated upon layer 12 is a layer 49 ofSN, and a layer 53 of chalcogenide material. Over layer 53 is anotherlayer 47 of carbon or titanium nitride, and upon that layer 47 isanother layer 49 of SiN. Finally, a metal layer 51 is situated upon thetop most layer 49 which is also composed of SiN. Metal layer 51 alsomakes contact through a contact hole in lower layer 49 with top mostlayer 47. Layer 53 also makes contact through a contact hole in lowerlayer 49 with lower layer 47.

In one alternative embodiment of the present inventive diode,programnable resistor 46 can be removed. In this embodiment, as shown inFIG. 9, metal contact 48 is secured directly to top portion 42 ofsilicon plug 38.

In yet another alternative embodiment, resistance through the inventivediode is decreased by lining diode trench 24 with a second refractorymetal silicide layer. As disclosed above with regard to vertical diode50, top portion 42 is heavily doped with the first type dopant. The useof a heavily doped top portion 42 of a diode, as opposed to a standarddoping, increases the rate of current flow through the diode in theforward bias direction. As a result of having a heavily doped topportion 42, however, bottom portion 44 of the diode must be lightlydoped so as to limit current leakage in the reverse bias direction. Ingeneral, a lighter doping will decrease the current leakage. As thedosage decreases, however, the resistance also increases. It istherefore desirable to design a structure that decreases the resistancethrough bottom portion 44 without increasing leakage.

As depicted in FIG. 10, after diode trench 24 is formed, as previouslydiscussed with regard to FIG. 4, but before amorphous silicon layer 36is deposited, a sacrificial polysilicon layer 30 is deposited on layeredwafer 10. Polysilicon layer 30 is deposited with good step coverage oninterior surface 25 of diode trench 24. Deposition of polysilicon layer30 is performed using conventional methods such as sputtering orchemical vapor deposition. It is preferred that polysilicon layer 30 bedeposited in a thickness ranging between about 200 Angstroms to about500 Angstroms.

As also shown in FIG. 10, once polysilicon layer 30 is deposited, arefractory metal layer 32 is subsequently deposited over polysiliconlayer 30. Refractory metal layer 32 preferably has a thickness rangingfrom about 500 Angstrorns to about 1000 Angstroms. Deposition ofrefractory metal layer 32 may be accomplished by sputtering, chemicalvapor deposition, or most other process by which metals are deposited.Refractory metal layer 32 is preferably formed of titanium (Ti),however, other refractory metals such as tungsten (W), tantalum (Ta),cobalt (Co), and molybdenum (Mo) can also be used.

Next, polysilicon layer 30 and refractory metal layer 32 are sintered soas to react together and from a single refractory metal silicide layer34 as shown in FIG. 11. Refractory metal silicide layer 34 has arelatively low contact resistance and is positioned so as to lineinterior surface 25 of diode trench 24. The composition of refractorymetal silicide layer 34 is dependent on the refractory metal used. WhereTi is used, refractory metal silicide layer 34 is TiSi₂. Other silicidesthat can be formed include, by way of example, WSi₂ TaSi₂, CoSi₂ andMoSi₂.

The sintering step is performed at a temperature ranging from about 500°C. to about 700° C., and an exposure time ranging between about 5seconds to about 20 seconds. Conventional heat treating processes, suchas RTP, can be used for the sintering. In the preferred embodiment,however, the heating does not need to be performed in a nitrogen richatmosphere since planarization will be performed using chemicalmechanical polishing.

Once refractory metal silicide layer. 34 is formed, amorphous siliconlayer 36 is deposited, as shown in FIG. 12, over refractory metalsilicide layer 34. Amorphous silicon layer 36 is deposited in the samemanner as discussed with regard to FIG. 5 and thus fills diode trench24. Using the same process steps as discussed with regard to FIG. 6,chemical-mechanical polishing is used to remove the portion of amorphoussilicon layer 36 and refractory metal silicide layer 34 above planarizedsurface 37 of insulation layer 20. The resulting configuration, asdisclosed in FIG. 13, shows silicon plug 38 being housed within diodetrench 24 and lined by refractory metal silicide layer 34.

Using the same method as previously discussed, the amorphous siliconused in amorphous silicon layer 36 and housed within diode trench 24 isheated to form large grain polysilicon. The preferred size of diodetrench 24 and the average diameter grain size of the polysilicon aresubstantially as previously disclosed.

With portions of amorphous silicon layer 36 removed, a protective andinsulative silicon layer 40 is deposited, as shown in FIG. 14, in ablanket over layered wafer 10 so as to span diode trench 24. Insulativesilicon layer 40 can be composed of either silicon dioxide or siliconnitride. Silicon layer 40 is preferably deposited in the same manner asdiscussed with insulation layer 20.

Shown positioned on top of silicon layer 40 is a photoresist layer 41.Photoresist layer 41 is patterned to mask silicon layer 40 so thatconventional etching can be performed to produce a passageway 56 thatextends through silicon layer 40 and exposes silicon plug 38 withindiode trench 24. Passageway 56 preferably has a width smaller than thewidth of silicon plug 38 and is centrally aligned on silicon plug 38 soas not to expose or contact refractory metal silicide layer 34.

Silicon plug 38 is then heavily doped through passageway 56 with thefirst type of dopant to form a top portion 52 of silicon plug 38, asshown in FIG. 14. Plug 38 is thus shown as comprising a “U” shapedbottom portion 54 being lightly doped with the second type of dopant.Top portion 52 is bounded within bottom portion 54 and is heavily dopedwith the first type of dopant. Top portion 52 is formed in the samemethod as discussed with respect to the formation of top portion 42 inFIG. 7. The difference between top portion 52 and top portion 42 is thattop portion 52 must be bounded by bottom portion 54 so as not to contactrefractory metal silicide layer 34.

Using substantially the same methods as discussed with regard to FIG. 8,a programmable resistor 46 is deposited over silicon layer 40 and withinpassageway 56 so as to contact top portion 52 of silicon plug 38.Finally, a metal contact 48 is positioned on programmable resistor 46 tocomplete a vertical diode 58 incorporating features of the presentinvention. As previously discussed however, programmable resistor 46 canbe eliminated if desired so that metal contact 48 directly contacts topportion 52 of silicon plug 38.

By lining diode trench 24 with refractory metal silicide layer 34, thearea of lightly doped bottom portion 54 is minimized. In turn,minimizing bottom portion 54 decreases the resistance through diode 58.The resistance is further decreased by the fact that the current flowsthrough refractory metal silicide layer 34 which has an extremely highconductance and thus low resistance.

In yet another alternative embodiment, a Schotkky diode can be formedincorporating features of the present invention. In general, a Schotkkydiode is formed by placing a metal in contact with a lightly dopedregion. To accomplish this, rather than doping silicon plug 38 to formtop portion 52, as discussed with regard to FIG. 14, a platinum silicide(PtSi₂) layer 60 is formed on the exposed surface of silicon plug 38, asshown in FIG. 16. Platinum silicide layer 60 is formed using the samemethods as discussed in the formation of refractory metal silicide layer34. Namely, a layer of sacrificial polysilicon is deposited over siliconplug 38. A layer of platinum is then deposited over the sacrificialpolysilicon. Sintering is then used to form the PtSi₂. In an alternativeembodiment, other refractory metals, such as those previously discussedwith regard to refractory metal silicide layer 34, can replace theplatinum and thus form alternative silicides.

An aqua regia process is next used to remove the non-reactive platinum.As shown in FIG. 17, the diode can then be finished by selectivelyattaching a programmable resistor 46 and a metal contact 48 aspreviously discussed.

As also shown in FIG. 17, to deliver a current to the above disclosedinventive diodes, a connection plug 62 is formed through insulationlayer 20 so as to contact refractory metal silicide layer 17. Connectionplug 62 is formed by initially etching a connection trench 64 having aninterior surface 65 through insulation layer 20. Connection trench 64has substantially the same configuration as diode trench 24 and ispreferably formed at the same time and in the same manner as diodetrench 24. The formation of diode trench 24 is as discussed with regardto FIG. 4.

Next, a titanium layer 66 is deposited on interior surface 65 ofconnection trench 64. Titanium layer 66 is deposited in the same manner,as discussed with regard to FIG. 10, that refractory metal layer 32 isdeposited over polysilicon layer 30. In one embodiment, titanium layer66 is exposed to a nitrogen rich environment at an elevated temperatureto convert the titanium to titanium nitride (TiN). Next, connectiontrench is filled with tungsten (W), using a deposition process, to forma tungsten plug 68.

Finally, a metal contact 70, preferably made of aluminum, is positionedto contact tungsten plug 68. In this configuration, an electricalcurrent delivered to metal contact 70, travels through connection trench64 and along active region 18 where it enters each of the connecteddiodes.

The present invention also discloses other embodiments of verticaldiodes that minimize resistance and current leakage. For example, anadditional embodiment of a vertical diode incorporating features of thepresent invention is disclosed in FIGS. 18 and 19. As disclosed in FIG.18, a silicon substrate 80 of a silicon wafer 81 has been overlaid by anoxide layer 82. Silicon substrate 80 is lightly doped with a first typedopant that is preferably a P-type dopant. Alternatively, of course,silicon substrate 80 can be doped with an N-type dopant. A conventionalmasking and etching process has been used to form a hole 84 throughoxide layer 82 to expose a surface 86 of silicon substrate 80.

A polysilicon layer 85 has been deposited in a blanket layer oversilicon wafer 81 so as to fill hole 84. Polysilicon layer 85 isdeposited in an open or closed deposition tube so as to simultaneouslybe heavily doped with a second type dopant. As shown in FIG. 19, a CMPor other planarizing step has been used to remove the portion ofpolysilicon layer 85 above oxide layer 82. As a result, a silicon plug88 is formed within hole 84.

Next, silicon wafer 81 is heated to an elevated temperature, such as byusing an RTP or tube furnace step, so as to diffuse a portion of thedoping ions from polysilicon plug 88 into silicon substrate 80, therebyforming an active region 90. The benefit conferred in doping bydiffusion is that such doping allows for shallow junction formation.Preferred process flow parameters for diffusion of the doping ions are aheat cycle of 30 minutes at 900° C. in an atmosphere of gaseous diatomicnitrogen within a batch processing tube furnace. As a result, a verticaldiode 91 is formed having a junction 93 formed at the interface ofactive region 90 and silicon substrate 80. If desired, a programmableresistor 87 and a metal contact 89 can be formed over polysilicon plug88 in substantially the same way that programmable resistor 46 and metalcontact 48 are formed over silicon plug 38 in FIG. 8.

In the above embodiment, junction 93 is formed within the single crystalstructure of silicon substrate 80 and thus has relatively low resistanceand low current loss. One problem with this configuration, however, isthat the dopants migrating from polysilicon plug 88 into siliconsubstrate 80 migrate both vertically and laterally. Accordingly, asshown in FIG. 19, active region 90 has a larger diameter than hole 84.This increase in size of active region 90 can create isolation problemswhen attempting to densely compact a plurality of vertical diodes 91 ina defined area. More specifically, if the adjacent diodes are formed tooclose together, a short can occur between adjacent active regions 90 asa voltage is applied to the diodes. To prevent shorts, the diodes mustbe placed further apart, thereby decreasing their formation density.

To remedy this isolation problem, the present invention also disclosesinventive diode configurations that maximize compaction and minimize thepossibility of shorting. The method for forming the below alternativeembodiment of an inventive diode is discussed as part of an integratedsystem for simultaneously forming a plurality of memory capable diodesthat have low series resistance. It is submitted, however, that thoseskilled in the art would be able to use the present disclosure toconstruct and use the diode portion of the system in any environmentwhere a diode is needed.

As shown in FIG. 20, the first step in formation of the inventive diodeis to use a local oxidation of silicon (LOCOS) process to grow a seriesof field oxide regions 92 on a silicon substrate 94 of a silicon wafer95. Silicon substrate 94 was initially doped with an N-type dopant andhas a series of exposed surfaces 96 positioned between each adjacentfield oxide region 92. Next, each exposed surface 96 is lightly doped byion implantation with P-type dopants to form active regions 98. Theconfiguration shown in FIG. 20 in which two active regions 98 and threeoxide lines 92 are shown is simply illustrative. In practice, any numberof active regions 98 and oxide lines 92 can simultaneously be formed onsilicon wafer 95.

FIG. 21 is a top view of a section of silicon wafer 95 showing theelements described above in FIG. 20. As shown in FIG. 21, oxide lines 92and active regions 98 each have a length extending along the surface ofsilicon wafer 95. As will be discussed later in greater detail, activeregions 98 act as digit lines that communicate with discrete diodesformed on active region 98. Once all of active regions 98 are doped,alternating portions of active region lines 98 are heavily doped with aP-type dopant. This is accomplished by using a layer of photoresist toinitially cover active regions 98. A conventional masking and etchingprocess is then used to expose those portion of active regions 98 thatare to be heavily doped. Ion implantation is then used to dope theexposed areas. With the layer of photoresist removed, FIG. 21 showsactive regions 98 as comprising alternating P plus active regions 100and P minus active regions 102.

FIG. 22 is a cross-sectional view of silicon wafer 95 taken across Pminus active region 102. As shown therein, a blanket oxide layer 104 hasbeen deposited over silicon wafer 95. As used in the specification andappended claims, the term “oxide layer” is interpreted to include alayer made out of any insulative silicon material, e.g. siliconmonoxide, silicon dioxide, and silicon nitride. Chemical mechanicalpolishing (CMP) or some other equivalent process has also been used toplanarize oxide layer 104 so as to form a smooth top surface 106.Deposited on top of top surface 106 is a silicon nitride layer 108 thatcan also be subjected to a CMP process. As will be discussed later,silicon nitride layer 108 functions as an etch stop for laterprocessing.

A conventional masking and etching process has next been used to formholes 110 that extend through silicon nitride layer 108, oxide layer104, and exposed surface 96 of P minus active regions 102. With holes110 formed, silicon wafer 95 is positioned in a reactor chamber and anepitaxial silicon layer 112 is grown exclusively on exposed surface 96of P minus active regions 102. Epitaxial silicon layer 112 is lightlydoped during growth with a P-type dopant. The growing of epitaxialsilicon is both a time consuming and expensive process. As such, it ispreferable to minimize the thickness of epitaxial silicon layer 112 soas to minimize the amount of epitaxial silicon that needs to be grown.As discussed in greater detail below, however, epitaxial silicon layer112 must be sufficiently thick to enable the formation of a junction forthe inventive diode. As such, it is preferable that epitaxial siliconlayer 112 have a thickness in a range between about 1500 Angstroms toabout 3000 Angstroms, with about 2000 Angstroms to about 2500 Angstromsbeing more preferred. Methods for forming epitaxial silicon layer 112are known in the art, but a preferred method for the forming is at atemperature of 950-1200° C. in an atmosphere of silane, SiH₂Cl₂, ordisilane, and the deposition method is LPCVD at 1000 Angstroms perminute. Alternatively, atmospheric pressure deposition can also beemployed.

Next, a polysilicon layer 111 has been deposited over silicon wafer 95so as to fill the remaining portion of each hole 110. Polysilicon layer111 is heavily doped during deposition with an N-type dopant. A CMPprocess is then used to planarize polysilicon layer 111 down to siliconnitride layer 108. As a result, FIG. 22A shows contact holes 110 beingfilled with lightly P doped epitaxial silicon layer 112 contacting Pminus active region 102 and a N doped polysilicon plug 114 positioned ontop of epitaxial silicon layer 112.

Silicon wafer 95 is next heated to an elevated temperature, such as byusing an RTP process, sufficient to cause a portion of the N-typedopants in polysilicon plug 114 to diffuse into a top portion 115 ofepitaxial silicon layer 112. As such, a diode is formed having ajunction 123, defined by the interface between a top portion 115 and abottom portion 117 of epitaxial silicon layer 112. Top portion 115 isdefined by the area that is N doped by the ions diffused frompolysilicon plug 114. Bottom portion 117 is the remaining area ofepitaxial silicon layer 112. As a result of epitaxial silicon having asingle crystal structure, current leakage and resistance is minimized atjunction 123. Furthermore, since junction 123 is isolated within hole110, similarly constructed diodes can be formed closer together atincreased density without fear of shorting.

FIG. 23 is a cross-sectional view taken along the length of one line ofactive regions 98. In the preferred embodiment, as shown in FIG. 23 andthe corresponding top view in FIG. 24, two adjacent holes 110 aresimultaneously formed within P minus active regions 102 according to theabove process. As such, two vertical diodes can simultaneously beformed. Likewise, after each polysilicon plug 114 is formed, aconventional masking and etching process can be used to form a hole 120in each of P plus active regions 100 on opposing sides of P minus activeregions 102. Holes 120 extend through silicon nitride layer 108 andoxide layer 104 and expose P plus active region 100. As shown in FIG.25, a polysilicon layer is then deposited over silicon wafer 95 so as tofill each of holes 120. The polysilicon layer is heavily doped duringdeposition with a P-type dopant. A CMP process is then used to removethe portion of the polysilicon layer above silicon nitride layer 108 sothat polysilicon plugs 122 are formed filling contact holes 120.

Once polysilicon plugs 122 are formed, a programmable resistor 116 canbe formed in contact with polysilicon plug 114 in the same manner thatprogrammable resistor 46 is formed in contact with polysilicon plug 38in FIG. 8. After programmable resistors 116 are formed, metal row lines118 are formed that span between active regions 98 to cover and contactaligned programmable resistors. Metal row lines 118 are formed byinitially depositing a metal layer over silicon wafer 95 so as to coverprogrammable resistors 116. A layer of photoresist is next depositedover the metal layer. A conventional masking and etching process is usedto remove the unwanted portion of the metal layer so that only the metalrow lines 118 connecting and covering programmable resistors 116 remain.The remaining photoresist material is then removed.

In FIG. 25A, a blanket oxide layer 124 is deposited over silicon wafer95 so as to cover metal row lines 118. A CMP step is used to planarizeoxide layer 124 so that a smooth surface 126 is obtained. To accesspolysilicon plugs 122, a layer of photoresist is deposited over surface126. Masking and etching steps are then used to form channels 128extending through oxide layer 124 and down to polysilicon plugs 122.Channel 28 has a diameter slightly larger than the diameter of contacthole 120 so that a portion of silicon nitride layer 108 is exposed.

A conductive material, such as any conventional metal, is next depositedin a blanket layer to fill channels 128 and interconnect polysiliconplugs 122 on opposing sides of the vertical diodes. This is preferablyaccomplished by depositing a titanium layer 130, or other refractorymetal, by the process of sputtering so that a thin layer is formed onthe interior surface of contact hole 120. Next, a layer 132 of tungstenis deposited by CVD methods so that a thin layer is deposited over layer130. Layer 130 is composed of TIN, titanium, or both TiN and titaniumlayer. A CMP step, or a etchback dry etch step, is then used to removelayer 130 and tungsten layer 132 that is not within contact hole 120.Finally, a blanket layer of tungsten is deposited over silicon wafer 95so as to fill the remaining area within channel 128, thereby providingstrapping over the vertical diodes.

The present invention also discloses other embodiments of low seriesresistance, vertical diodes that incorporate strapping. In one suchembodiment, as shown in FIG. 26, vertical diodes are formed on a siliconwafer 137 by initially lightly implanting a P-type dopant in an N dopedsilicon substrate 138 to form an active region 136. Active region 136comprises a digit line that is bounded on opposing sides by field oxide140.

As shown in FIG. 27, a polysilicon layer 142 is next deposited in ablanket layer over silicon wafer 137. Polysilicon layer 142 is heavilydoped by ion implantation with an N-type dopant. A photoresist layer 199is next deposited over polysilicon layer 142 and field oxide 140.Conventional masking and etching steps are then used to form holes 144through photoresist 142 that expose select portions of polysilicon layer142. P-type dopants are then implanted through holes 144 so as toheavily dope portions of polysilicon layer 142. As such, polysiliconlayer 142 is shown as having heavily N doped regions 146 and heavily Pdoped regions 148.

Once photoresist layer 199 has been removed, an additionalphotolithography step is used to selectively remove portions ofpolysilicon layer 142 so that a plurality of heavily N doped columns 150and heavily P doped columns 152, corresponding respectively to N dopedregions 146 and P doped regions 148, project from active region 136.Silicon wafer 137 is next heated to an elevated temperature, such as byan RTP step, so as to partially diffuse the dopant ions within columns150 and 152 into the active region 136, thereby forming infused regions154 below columns 150 and infused regions 155 below columns 152. As aresult, vertical diodes are formed that have a junction 153 at theinterface between silicon substrate 138 and infused regions 155.

As shown in FIG. 29, a blanket silicon oxide layer 156, i.e., siliconmonoxide or silicon dioxide, is next deposited over the silicon wafer137 so as to cover columns 150 and 152. A photolithography process isused to etch channels 158 through silicon oxide layer 156 down to eachof the columns 150 and 152. A refractory metal silicide layer 160 and arefractory metal nitride layer 161 are next formed on the interiorsurface of each of the channels 158. Refractory metal silicide layer 160is formed by initially depositing a refractory metal layer over siliconoxide layer 156 so that the interior surface of channels 158 are linedwith the refractory metal layer. Deposition of the refractory metallayer may be accomplished by sputtering, chemical vapor deposition, ormost other process by which metals are deposited. The refractory metallayer is preferably formed of titanium (Ti), however, other refractorymetals such as tungsten (W), tantalum (Ta), cobalt (Co), and molybdenum(Mo) can also be used.

Next, an RTP step is used to sinter the refractory metal layer. Thesintering step is performed in a nitrogen (N₂) rich environment at atemperature ranging from about 500° C. to about 650° C. The preferredexposure time ranges between about 10 seconds to about 20 seconds.

As a result of the sintering, the top or exposed portion of therefractory metal layer reacts with the surrounding nitrogen to formrefractory metal nitride layer 161, for example, TiN. In contrast theportion of the refractory metal layer adjacent to silicon oxide layer156 and columns 150 and 152, reacts with the polysilicon to formrefractory metal silicide layer 160. The composition of refractory metalsilicide layer 160 is dependent on the refractory metal used. Where Tiis used, refractory metal silicide layer 160 is TiSi₂. Other suicidesthat can be formed include, by way of example, WSi₂ TaSi₂, CoSi₂, andMoSi₂.

A tungsten layer is next deposited in a blanket over silicon wafer 137so as to fill the remaining portion of each of channels 158. A CMPprocess is next used to planarize the surface of the silicon wafer downto the oxide 156. As a result, each of channels 158 is filled with atungsten plug 162 bounded by a refractory metal nitride layer 161 and arefractory metal silicide layer 160.

As shown in FIG. 30, a programmable resistor 164 can be positioned incontact with each of the tungsten plugs 162 over the N plus columns 150in the same manner that programmable resistor 46 is formed in contactwith polysilicon plug 38 in FIG. 8. A blanket metal layer can next bedeposited over silicon wafer 137 and then patterned so as to form metalcontact lines 166 contacting and covering programmable resistors 164.

A second blanket oxide layer 168 is next deposited so as to cover metalcontact lines 166. The photolithography process is then used to formchannels 170 through oxide layer 168 down to tungsten plugs 162 above Pplus columns 152. A refractory metal silicide layer 172, refractorymetal nitride layer 174, and tungsten plug 176 are next positionedwithin each channel 170 in the same way that they are positioned inchannel 158. Finally, an aluminum line is deposited in a blanket layerover silicon wafer 137. A patterning step is then used to form contactline 178 that communicates with each of tungsten plugs 176.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by united states letterspatent is:
 1. A method for forming a diode on a silicon wafer having asilicon substrate, the method comprising: lightly doping a portion ofthe silicon substrate with a second type dopant to form an activeregion; forming an oxide layer over the silicon substrate; etching ahole through the oxide layer to expose a portion of the active region;growing an epitaxial silicon layer that is lightly doped with the secondtype dopant, the epitaxial silicon layer being grown on the activeregion within the hole; filling the hole with a polysilicon plug that isheavily doped with a first type dopant; and performing a heat process topromote solid phase diffusion of the first type dopant from thepolysilicon plug into the epitaxial silicon layer.
 2. A method asrecited in claim 1, further comprising doping with the first type dopantthe silicon substrate.
 3. A method as recited in claim 1, wherein theepitaxial silicon layer has a top portion and a bottom portion, and theheat process is performed to promote solid phase diffusion of the firsttype dopant from the polysilicon plug into the top portion of theepitaxial silicon layer.
 4. A method as recited in claim 1, wherein thefirst type dopant comprises a P-type dopant.
 5. A method as recited inclaim 1, wherein the second type dopant comprises an N-type dopant andwherein forming an oxide layer comprises depositing an oxide layer.
 6. Amethod as recited in claim 1, further comprising: forming a programmableresistor in communication with the conductive plug; and constructing ametal contact in communication with the programmable resistor.
 7. Amethod as recited in claim 1, wherein the first type dopant provides aconductivity opposite to the conductivity provided by the second typedopant.
 8. A method as recited in claim 1, wherein performing a heatprocess comprises an RTP step.
 9. A method as recited in claim 1,further comprising depositing a silicon nitride layer over the oxidelayer, doping with the first type dopant the silicon substrate, andforming an oxide layer comprises depositing the oxide layer.
 10. Amethod for forming a diode on a silicon wafer having a silicon substratedoped with a first type dopant, the method comprising: lightly doping aportion of the silicon substrate with a second type of dopant to form anactive region; depositing an oxide layer over the silicon substrate;etching a bole through the oxide layer to expose a portion of the activeregion; growing an epitaxial silicon layer that is lightly doped withthe second type dopant, the epitaxial silicon layer being grown on theactive region within the hole and having a top portion and a bottomportion; filling the hole with a polysilicon plug that is heavily dopedwith the first type of dopant; and heating the silicon wafer to atemperature sufficient to diffuse the dopants from the polysilicon pluginto the top portion of the of the epitaxial silicon layer.
 11. A methodas recited in claim 10, further comprising: forming a programmableresistor in communication with the conductive plug; and p1 constructinga metal contact in communication with the programmable resistor.
 12. Amethod as recited in claim 10, wherein the method further comprisesdepositing a silicon nitride layer over the oxide layer.
 13. A method asrecited in claim 10, wherein heating the silicon wafer comprises an RTPstep.
 14. A method as recited in claim 10, wherein the first type dopantprovides a conductivity opposite to the conductivity provided by thesecond type dopant.
 15. A method as recited in claim 10, wherein thefirst type dopant comprises a P-type dopant.
 16. A method as recited inclaim 10, wherein the second type dopant comprises an N-type dopant. 17.A method for forming a diode on a silicon wafer having a siliconsubstrate, the method comprising: doping with a first type dopant thesilicon substrate; lightly doping a portion of the silicon substratewith a second type dopant to form an active region, wherein the secondtype dopant provides a conductivity opposite to the conductivityprovided by the first type dopant; depositing an oxide layer over thesilicon substrate; etching a hole through the oxide layer to expose aportion of the active region; growing an epitaxial silicon layer that islightly doped with the second type dopant, the epitaxial silicon layerbeing grown on the active region within the hole and having a topportion and a bottom portion; filling the hole with a polysilicon plugthat is heavily doped with the first type dopant; heating the siliconwafer to cause diffusion of dopants from the polysilicon plug into thetop portion of the epitaxial silicon layer.
 18. A method as recited inclaim 17, further comprising: forming a programmable resistor incommunication with the conductive plug; and constructing a metal contactin communication with the programmable resistor.
 19. A method as recitedin claim 17, wherein the first type dopant comprises a P-type dopant.20. A method as recited in claim 17, wherein the second type dopantcomprises an N-type dopant.